Exhaust gas purifying apparatus

ABSTRACT

An exhaust gas purifying apparatus includes a primary diesel particulate filter provided in an exhaust line of a diesel engine, a secondary exhaust line branched from the exhaust line at an upstream side of the primary diesel particulate filter, a secondary diesel particulate filter provided in the secondary exhaust line, and a low pressure part provided in the secondary exhaust line at a downstream side of said secondary diesel particulate filter. The secondary diesel particulate filter has a soot load capacity smaller than the soot load capacity of the primary diesel particulate filter. The low pressure part provides a pressure lower than a pressure of the branching point. The apparatus further includes a differential pressure measuring part for measuring a differential pressure between an inlet and an outlet of the secondary diesel particulate filter.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to EuropeanPatent Application No. 06386032.4 filed on Oct. 17, 2006 and JapanesePatent Application 2007-209681 filed on Aug. 10, 2007. The contents ofthis European Patent application and this Japanese Patent applicationare incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to exhaust gas purifying apparatuses.

2. Discussion of the Background

Conventionally, a diesel particulate filter (DPF) of porous ceramic hasbeen used for collecting particulate matter (PM) primarily of C (carbon)emitted from a diesel engine. With such a diesel particulate filter,there occurs gradual deposition of particulate matter with continual usethereof, and thus, it has been practiced in the art of exhaust gaspurifying apparatus that uses a diesel particulate filter to remove thedeposited particulate matter by causing a burning process inside thediesel particulate filter periodically and regenerate the dieselparticulate filter.

It is preferable that such regeneration of the diesel particulate filteris conducted during the operation of the diesel engine, withoutreplacing or dismounting the filter, and thus, it is practiced in theart to carry out fuel injection in the state that the piston is movingdown in the cylinder following combustion to form a high temperature gas(post injection process). Thereby, the deposited particulate matter isburned with the high temperature gas thus formed.

FIG. 1 shows the overall construction of a conventional exhaust gaspurifying system of a diesel engine equipped with a diesel particulatefilter according to a related art of the present invention.

With the conventional exhaust gas purifying system explained withreference to FIG. 1, it should be noted that such regeneration of filteris conducted each time the vehicle has traveled a predetermined mileagesuch as 500 km, over the duration of 10 minutes, for example.

In the case the filter regeneration by way of post injection has beenconducted impartially, the regeneration is carried out irrespective ofactual amount of collection of the particulate matter in the filter.Thus, in order to ensure that there occurs no excessive deposition ofthe particulate matter in the filter, there is a need to set theinterval of filter regeneration to be shorter than what is actuallyneeded for the sake of safety.

On the other hand, there is a known construction of carrying outregeneration of the diesel particulate filter 12B by way of positinjection as shown in FIG. 3, in which a differential pressure ΔP ismeasured between the upstream side and downstream side of the dieselparticulate filter 12B and the posit injection is carried out when theforegoing differential pressure ΔP has reached a predetermined value.Reference should be made to the U.S. Pat. No. 6,952,920.

Further, U.S. Pat. No. 5,651,248 describes the construction that uses,in addition to the diesel particulate filter, a detection filter andevaluates the amount of the particulate matter collected in thedetection filter by measuring the electric resistance. According to thistechnology, the particulate matter collected by the diesel particulatefilter and the particulate matter collected by the detection filter aresubjected to burning by using a heater when the detected resistance hasdecreased below a predetermined value. With this, regeneration of filteris achieved.

As shown in FIGS. 5A, 5B, and 5C, there can be a situation in which thethickness of the collected particulate matter changes in spite of thefact that the deposition amount thereof is the same. Now, when thethickness of the collected particulate matter is different, it becomesdifficult to measure the electrical resistance precisely, and theretends to be caused error in the evaluation of the deposition amount.

Further, in the case there is caused a deposition of ash in the dieselparticulate filter or detection filter after burning of the particulatematter, no precise measurement of electrical resistance is possibleanymore and there tends to be caused a large error in the evaluation ofthe deposition amount.

Further, with the use of the detection filter, there is causeddegradation in the filter or electrode with time or with use in theambient of exhaust gas. Particularly, the electrode (terminal formed ofa conductive metal) is formed by infiltrating a metal such as Cu, Cr,Ni, or the like, and thus, there is a tendency of causing problems ofphysical degradation, oxidation degradation and thermal degradation,such as oxidation, adhesion of impurities, cracking, corrosion, and thelike.

When there is caused degradation in the filter or electrode, it is nolonger possible to carry out precise measurement of the electricresistance and error is tend to be caused in the evaluation of thedeposition amount of the particulate matter.

The contents of U.S. Pat. Nos. 6,952,920 and 5,651,248 are incorporatedherein by reference in their entirety.

SUMMARY OF THE INVENTION

An exhaust gas purifying apparatus of the present invention includes aprimary diesel particulate filter provided in a primary exhaust line ofa diesel engine; a secondary exhaust line branched from the primaryexhaust line from a branching point located at an upstream side of theprimary diesel particulate filter; a secondary diesel particulate filterprovided in the secondary exhaust line, the secondary diesel particulatefilter having a soot storage capacity smaller than the soot storagecapacity of the primary diesel particulate filter; a low pressure partprovided in the secondary exhaust line at a downstream side of thesecondary diesel particulate filter, the low pressure part providing apressure lower than a pressure of the branching point; and adifferential pressure measuring part measuring a differential pressurebetween an inlet and an outlet of the secondary diesel particulatefilter.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram showing an overall engine system that uses aconventional exhaust gas purifying apparatus;

FIG. 2A is a diagram showing a schematic construction of a dieselparticulate filter;

FIG. 2B is a diagram showing a constituting element of the dieselparticulate filter;

FIG. 2C is a diagram showing the operational principle of the dieselparticulate filter;

FIG. 2D is a diagram showing the state of the particulate mattercollected by the diesel particulate filter;

FIG. 3 is a diagram showing the overall construction of a conventionalengine system that uses an exhaust gas purifying apparatus according toa related art of the present invention;

FIG. 4A is a diagram explaining the problem with the exhaust gaspurifying apparatus of FIG. 3, FIG. 4B is a diagram of collectedparticulate matter at location A1 in FIG. 4A, FIG. 4C is a diagram ofcollected particulate matter at location B2 in FIG. 4A, and FIG. 4D is adiagram of collected particulate matter at location C3 in FIG. 4A;

FIGS. 5A, 5B, and 5C are other diagrams explaining the problem of theexhaust gas purifying apparatus of FIG. 3;

FIG. 6 is a diagram showing the construction of an exhaust gas purifyingapparatus according to a first embodiment of the present invention;

FIG. 7A is a diagram showing the construction of a secondary dieselparticulate filter used in FIG. 6;

FIG. 7B is a diagram explaining the principle of the secondary dieselparticulate filter of FIG. 7A;

FIG. 8 is a diagram showing the construction of a particulate matter(PM) sensor that uses the secondary diesel particulate filter of FIG. 6;

FIG. 9 is a diagram explaining the effect of the embodiment of theinvention;

FIG. 10 is a flow chart explaining the regeneration operation of thediesel particulate filter in the exhaust gas purifying apparatusaccording to a second embodiment of the present invention; and

FIG. 11 is a flowchart explaining another regeneration operation of thediesel particulate filter of the exhaust gas purifying apparatusaccording to the second embodiment of the present invention;

FIG. 12 is a diagram showing the overall construction of the dieselengine that includes the exhaust gas purifying apparatus of FIG. 6.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

According to an embodiment of the present invention, an exhaust gaspurifying apparatus includes a primary diesel particulate filterprovided in a primary exhaust line of a diesel engine; a secondaryexhaust line branched from the primary exhaust line from a branchingpoint located at an upstream side of the primary diesel particulatefilter; a secondary diesel particulate filter provided in the secondaryexhaust line, the secondary diesel particulate filter having a sootstorage capacity smaller than the soot storage capacity of the primarydiesel particulate filter; a low pressure part provided in the secondaryexhaust line at a downstream side of the secondary diesel particulatefilter, the low pressure part providing a pressure lower than a pressureof the branching point; and a differential pressure measuring partmeasuring a differential pressure between an inlet and an outlet of thesecondary diesel particulate filter.

Preferably, the secondary exhaust line has a downstream end part at thedownstream side of the secondary diesel particulate filter such that thedownstream end part is connected to an air intake part of the dieselengine.

Preferably, the downstream end part is connected to an upstream side ofan air filter.

Preferably, the secondary exhaust line has a downstream end part at thedownstream side of the secondary diesel particulate filter such that thedownstream end part is connected to the primary exhaust line at adownstream side of the primary diesel particulate filter.

Preferably, the secondary exhaust line has a downstream end part at thedownstream side of the secondary diesel particulate filter such that thedownstream end part is connected to an exhaust gas recirculation line ofthe diesel engine.

Preferably, the secondary exhaust line further includes a flow meter orequivalent meter (e.g. a gas velocity meter).

Preferably, the secondary exhaust line further includes a temperaturemeasuring part.

Preferably, the secondary diesel particulate filter includes a heater.

Preferably, the secondary exhaust line includes a valve that maintains aflow rate of the exhaust gas flowing therethrough at a predeterminedvalue.

Preferably, the exhaust gas purifying apparatus further includes aholder, and wherein at least one of the differential pressure measuringpart, the temperature measuring part, the secondary diesel particulatefilter and the flow meter or equivalent meter (e.g. a gas velocitymeter) is accommodated in the holder.

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the drawings.

Referring to FIG. 1, a diesel engine 11 has an exhaust line 12, whereinthere is provided a diesel particulate filter 12B in the exhaust line 12for collecting the particulate matter contained in the exhaust gas andemitted from the diesel engine 11.

FIG. 2A shows the outline of the diesel particulate filter 12B whileFIG. 2B shows an element that constitutes the diesel particulate filter.

The diesel particulate filter 12B is formed of a filter unit 12A of aporous ceramic, typically of SiC, wherein there are formed a largenumber of gas passages 12 a in the filter unit 12A so as to extend fromone end to the other end thereof with a cross-section of 1 mm×1 mm, forexample.

Thereby, the diesel particulate filter 12B is formed by binding pluralfilter units (filter elements) 12A by a seal material (adhesion layer)and machining the peripheral part thereof such that the filter 12B as awhole has a cylindrical form. Further, the peripheral surface of thefilter 12B is covered by a seal material (coating layer). There may be acase in which only one unit 12A is used in the diesel particulate filter12B.

FIG. 2C shows the principle of the diesel particulate filter 12B.

As shown schematically in FIG. 2C, the plural gas passages 12 a havetheir upstream ends or downstream ends closed alternately with regard tothe direction of the exhaust gas flow from the engine, and the exhaustgas introduced to one such gas passage 12 a passes to an adjacent gaspassage by way of penetration through the porous member 12 b of thefilter 12B. Thereby, the particulate matter contained in the exhaust gasis collected by the porous member 12 b as the exhaust gas penetratestherethrough, and there is caused deposition of the particulate matter12 c on the porous member 12 b in the form of layer as shown in FIG. 2D.

Because the diesel particulate filter 12B thus causes deposition of theparticulate matter contained in the exhaust gas therein, there is a needof regenerating the filter with suitable timing by conducting a cleaningprocess (burning of the deposited particulate matter), as describedpreviously.

According to the conventional construction of FIG. 3, the regenerationof the diesel particulate filter 12B is carried out only when thedifferential pressure between the upstream side and the downstream sidehas reached the predetermined value, and unnecessary post injectionprocess is suppressed. Thereby, the fuel efficiency of the vehicledriven with the diesel engine is improved.

Unfortunately, collection of the particulate matter in the dieselparticulate filter 12B is not uniform. As shown in FIG. 4A, there is adifference of density or thickness in the collected particulate matterdepending on the locations (A,1)(shown in FIG. 4B), (B,1), (C,1), (A,2),(B,2)(shown in FIG. 4C), (C,2), (A,3), (B,3), (C,3)(shown in FIG. 4D) inthe filter 12B. Further, it can be seen that there is formed a cavity inthe layer of the deposited particulate matter, wherein such a cavityformed in the layer of particulate matter provides as a local passage ofexhaust gas. Existence of such a cavity indicates occurrence ofuncontrolled burning in the collected particulate matter and indicatesfurther that there has been caused local burning in the collectedparticulate matter.

Further, as shown in FIGS. 5A, 5B, and 5C, the density of the collectedparticulate matter can take different values even when the depositionamount of the particulate matter is identical. FIGS. 5A, 5B, and 5C showthat there is caused a large variation in the differential pressureaccording to the change of the thickness, even when the depositionamount is identical. In the examples of FIGS. 5A, 5B, and 5C, forexample, it should be noted that the deposition amount of theparticulate matter is 8 g/L throughout. In spite of this, it can be seenin FIGS. 5A, 5B, and 5C that the differential pressure has changed from15.3 kPa to 8.8 kPa when the thickness of the collected particulatematter has changed from 109 μm to 255 μm. Thus, it can be seen thatthere is caused about twice as large difference in the differentialpressure.

Thus, when such non-uniform deposition or local cavity formation iscaused in the particulate matter 12 c collected in the conventionalconstruction of FIG. 3, there can be caused an error of as much as +50%with regard to the evaluation of the actually deposited particulatematter and the differential pressure ΔP, with regard to theoreticalcalculation values. As a result of such an error, there is caused alarge deviation in the relationship between the amount of the actuallydeposited particulate and the timing of regeneration. Further, in viewof the fact that the exhaust gas pressure and the exhaust gas flow ratechange with engine load or engine revolution, it is extremely difficultwith the construction of FIG. 3 to detect the deposition amount of theparticulate matter in the diesel particulate filter 12B precisely.

On the other hand, this U.S. Pat. No. 5,651,248 has a drawback in that,in addition to the problem that the construction thereof becomes complexbecause of the need of providing a heater in the diesel particulatefilter, there occurs electric power consumption at the time ofregeneration of the diesel particulate filter. In order to save theelectric power consumption at the time of filter regeneration, thetechnology of U.S. Pat. No. 5,651,248 selects the timing of executingthe filter regeneration such that the regeneration operation isconducted at the time the temperature of the diesel particulate filteris higher than a predetermined temperature, except for the case in whichthe diesel particulate filter is in the critical state with regard tothe deposition of the particulate matter and it is inevitable to carryout regeneration immediately. As a result, there is imposed arestriction on the timing of regenerating operation with thistechnology, and the degree of freedom of regenerating operation of theparticulate detection filter is restricted.

Further, with the technology of the U.S. Pat. No. 5,651,248, it is notpossible to use the diesel particulate filter during the regenerationoperation carried out by the heater, and because of this, there isprovided a reserve diesel particulate filter and switches to thisreserve diesel particulate filter during the regeneration process.However, such a construction requires two equivalent diesel particulatefilters together with a switching valve, and there arises a problem inthat the construction of the exhaust gas purifying apparatus becomesbulky. It is difficult to mount such an exhaust gas purifying apparatuson compact vehicles.

Further, with the technology of the U.S. Pat. No. 5,651,248,regeneration of the detection filter is carried out concurrently withthe diesel particulate filter or consecutively to the diesel particulatefilter, while such a construction cannot choose the timing ofregeneration of the detection filter arbitrarily, and there is a problemthat error tends to be caused in the regeneration timing of the dieselparticulate filter, depending upon the state of the detection filter.

When regeneration of the diesel particulate filter and regeneration ofthe detection filter are carried out independently, there is caused adecrease of ventilation resistance in the detection filter uponregeneration thereof, and the exhaust gas starts to flow primarilythrough the detection filter. Thereby, there is caused an error in thedetection of regeneration timing of the diesel particulate filter. Fromthese reasons, the technology of U.S. Pat. No. 5,651,248 carries out theregeneration of the detection filter and the regeneration of the dieselparticulate filter in synchronization as explained before.

Further, the technology of the U.S. Pat. No. 5,651,248 has a drawback inthe points of: (a) ash deposition; and (b) large evaluation error causedby deterioration.

Further, with the technology of the U.S. Pat. No. 5,651,248, therearises another problem from the very principle thereof of measuringelectric resistance of electrode for evaluating the deposition amount ofthe collected particulate matter.

According to the embodiment of the present invention, it becomespossible to measure the deposition amount of particulate matter in theprimary diesel particulate filter simply and easily, by using thesecondary diesel particulate filter of small capacity and hence lessprone to cause non-uniform deposition of the particulate matter and bydetecting the deposition of the particulate matter in the primary dieselparticulate filter by measuring the differential pressure occurring insuch a secondary diesel particulate filter. Thereby, it becomes possibleto suppress deterioration of fuel efficiency by excessive postinjection. Further, with the embodiment of the present invention, itbecomes possible to execute the regeneration of the secondary dieselparticulate filter independently to the primary diesel particulatefilter, and it becomes possible to constantly and precisely measure thedeposition amount of the particulate matter in the primary dieselparticulate filter by using the secondary diesel particulate filter.Further, it becomes possible to perform precise measurement whileeliminating the effect of ash deposition or degradation of the filter orelectrode.

Thereby, it becomes possible with the embodiment of the presentinvention to ensure supplying of the exhaust gas to the secondary dieselparticulate filter by connecting the downstream end of the secondarydiesel particulate filter to a low pressure part of the diesel engine.It should be noted that such a low pressure part may be located at anyof the low pressure part in the air intake system or exhaust system ofthe diesel engine.

Further, with the embodiment of the present invention, it becomespossible to avoid concentration of the exhaust gas of the exhaust line21 to the secondary exhaust line 21A with regeneration of the secondarydiesel particulate filter, which is caused as a result of decrease ofventilation resistance of the second exhaust line 21A with theregeneration of the secondary diesel particulate filter, by providing avalve in the secondary exhaust line and controlling the flow ratetherein to be constant. Thus, collection of the particulate matter inthe primary diesel particulate filter is caused similarly to thesecondary diesel particulate filter, and it becomes easier to avoid thedeviation caused between the evaluation of the deposition amount of theparticulate matter in the primary diesel particulate filter, carried outby the measurement of differential pressure in the secondary dieselparticulate filter, and the actual deposition amount of the particulatematter in the primary diesel particulate filter.

FIRST EMBODIMENT

FIG. 6 shows the construction of an exhaust gas purifying apparatus 20according to a first embodiment of the present invention.

Referring to the embodiment of FIG. 6, an exhaust gas from a dieselengine not illustrated is caused to flow into a primary dieselparticulate filter (DPF) 22 similar to the one explained previously withreference to FIG. 2A via an exhaust line 21, and the primary dieselparticulate filter (DPF) 22 collects the particulate matter in theexhaust gas as explained with reference to FIGS. 2C and 2D.

Further, with the construction of the embodiment of FIG. 6, a secondaryexhaust line 21A is branched from the exhaust line 21 from an upstreamside of the primary diesel particulate filter (DPF) 22, and a secondarydiesel particulate filter 22A is provided to the secondary exhaust line21A with a volume smaller than the volume of the primary dieselparticulate filter (DPF) 22. Further, there is provided a differentialpressure gauge 22B for measuring a differential pressure ΔP causedbetween an inlet and an outlet of the secondary diesel particulatefilter 22A. Further, with the construction of FIG. 6, there are provideda flow meter 24 and a control valve 23 in the secondary exhaust line 21Aat a downstream side of the secondary diesel particulate filter 22A,wherein the control valve 23 is used for maintaining the flow rate ofthe exhaust gas in the secondary exhaust line 21A constant based on themeasurement made by the flow meter 24. It should be noted that thecontrol valve 23 and the flow mater 24 may be provided anywhere on thesecondary exhaust line 21A. Here, it should be noted that the secondarydiesel particulate filter 22A, the differential pressure gauge 22B andthe flow meter 24 constitutes together a particulate matter (PM) sensorthat measures the amount of particulate contained in the exhaust gas.The particulate matter (PM) sensor may be defined to include atemperature measuring part (T1). Further, it is possible to provide atemperature measurement part T2 in the primary diesel particulate filter(DPF) 22.

It should be noted that the temperature measuring part in the exhaustline may be provided in ay of: (1) interior of the primary dieselparticulate filter, (2) interior of the secondary diesel particulatefilter, (3) in a pipe connected thereto, (4) exterior of the primarydiesel particulate filter, or (5) exterior of the secondary dieselparticulate filter. From the viewpoint of precise measurement of theexhaust gas temperature, the arrangement of (1) or (2) is preferable,wherein the arrangement of (2) is thought more preferable.

In the embodiment of FIG. 6, the primary diesel particulate filter (DPF)22 is formed of a porous ceramic of SiC, or the like having a porosityof about 35 to about 65% in the form of a honeycomb structure, whereinit can be seen that there are formed gas passages of a rectangularcross-section having a length of 1.1 mm, for example, for each edge inthe cross-section taken perpendicular to the gas flow direction, incorrespondence to the gas passages 12 a of FIG. 2B, wherein the gaspassages are arranged with a mutual separation of about 0.3 mm and formtogether a lattice pattern.

FIG. 7A shows the overall construction including the secondary dieselparticulate filter 22A, while FIG. 7B shows the principle of thesecondary diesel particulate filter 22A.

It should be noted that the secondary diesel particulate filter 22A maybe formed of a porous ceramic similar to the primary diesel particulatefilter (DPF) 22. In the case the secondary diesel particulate filter isformed of a porous ceramic, it is preferable that the secondary dieselparticulate filter includes a cell 22 b of a rectangular form. Therein,there is formed a single gas passage 22 a having a volume of about 65 mlor less such as about 0.05 to about 65 ml, or about 5% or less such asabout 0.05 to about 5% of the total volume of the exhaust gas passages(corresponding to passage 12 a of FIG. 3) in the primary dieselparticulate filter (DPF) 22. Alternatively, the gas passage 22 a mayhave a filtration area of about 0.1 to about 1000 cm² (preferably about1 to about 10 cm²). The gas passage 22 a may have a rectangularcross-sectional shape, for example, and is formed in the state that oneend thereof is closed (rear end is closed in the case of a cell). Here,it should be noted that the outer shape of the gas passage 22 a or theouter shape of the secondary diesel particulate filter 22A (cell 22 b)is not necessarily be identical to the cross-sectional shape of the gaspassages of the primary diesel particulate filter (DPF) 22, and thus,they can be shaped to any arbitrary shape of circular, square,octahedral, elliptical, or the like. Further, it should be noted thatthe porous ceramic constituting the secondary diesel particulate filter22A (cell 22 b) is not necessarily be identical with the porous ceramicthat forms the primary diesel particular filter (DPF) 22. Further, itshould be noted that the secondary diesel particulate filter 22A (cell22 b) may be formed of a material other than ceramics.

By forming the gas passage 22 a with the volume of about 5% or less ofthe exhaust gas passage (corresponds to the passage 12 a of FIG. 3) inthe primary diesel particulate filter (DPF) 22, or with the volume ofabout 65 ml or less, or with the filtration area of about 0.1 to about1000 cm² (preferably about 1 to about 10 cm²), it becomes possible tomeasure the deposition amount of the particulate matter in the primarydiesel particulate filter (DPF) 22 with a simple procedure.

The cell 22 b is provided with a temperature measuring part formeasuring the exhaust gas temperature T, and a thermocouple 22 d isprovided for the temperature measuring part. Further, a heater 22 h iswound around the cell 22 b for incinerating a soot layer 22 c depositedon the inner wall surface and regenerating the secondary dieselparticulate filter 22A. Further, the cell 22 b, the thermocouple 22 dand the heater 22 h are accommodated in a cylindrical holder 22 e ofSiO₂—Al₂O₃, or the like, by interposing an insulator 22 i of Al₂O₃, orthe like, and there is provided a diaphragm pressure gauge 22B in theholder 22 e for measuring the differential pressure ΔP, in such a mannerthat the exhaust gas in the secondary exhaust line 21A is supplied tothe pressure gauge 22B. The holder 22 e is accommodated in a metalhousing and is provided to the secondary exhaust line as the particulatematter (PM) sensor. The holder 22 e may also be provided inside the pipeof the secondary exhaust line or may be provided inside the secondaryexhaust line in the state accommodated in the metal housing.

Thus, when the exhaust gas in the secondary exhaust line 21A isintroduced to the exhaust passage 22 a of the cell 22 b, the exhaust iscaused to flow outside the cell through the wall surface of the cell 22b, and the particulate matter in the exhaust gas is collected similarlyto the case of FIG. 2C. Thereby, the particulate matter deposits on theinner surface of the cell 22 b to form a layer 22 c.

With the present embodiment, the deposition amount of the particulate 22c thus collected and deposited on the inner wall surface of the dieselparticulate filter 22 is calculated from the pressure difference ΔP andthe temperature T and flow rate Q of the exhaust gas thus obtained byusing the equation (1) below.

FIG. 8 shows a more detailed construction of the secondary dieselparticulate filter 22A of FIG. 6.

Referring to FIG. 8, the exhaust gas in the secondary exhaust line 21Ais supplied to the gas passage 22 a in the cell 22 b as represented byan arrow and is discharged, after passing through the cell, in thelateral direction or rear direction. Thereby, the heater 22 h on thecell 22 b is driven by the electric power supplied by a drive line 22 b1 and causes incineration in the particulate matter 22 c collected bythe cell 22 b. Further, the output signal of the diaphragm pressuregauge 22B is supplied to a control circuit via a signal line 22 p.

With the secondary diesel particulate filter 22A of FIGS. 7A and 7B, theamount of soot load of the particulate matter collected in the secondarydiesel particulate filter is calculated according to an equation of theformΔP=function (Flow, Temperature, Soot load, Geometry)with a preferred example shown below (although other expressions can bealso employed) according to which the thickness W[m] of a layer of theparticulate matter collected in the secondary diesel particulate filteris calculated according to

$\begin{matrix}{{\Delta\; P} = {{\frac{\mu\; Q}{2V_{trap}}{\left( {\alpha + W_{s}} \right)^{2}\left\lbrack {\frac{W_{s}}{K_{w}\alpha} + {\frac{1}{2K_{SOOT}}{\ln\left( \frac{\alpha}{\alpha - {2W}} \right)}} + {\frac{4\;{FL}^{2}}{3}\left( {\frac{1}{\left( {\alpha - {2W}} \right)^{4}} + \frac{1}{\alpha^{4}}} \right)}} \right\rbrack}} + {\frac{\rho\;{Q^{2}\left( {\alpha + {Ws}} \right)}^{4}}{V_{trap}^{2}}\left\lbrack {\frac{\beta\;{Ws}}{4} + {2{\zeta\left\lbrack \frac{L}{\alpha} \right\rbrack}^{2}}} \right\rbrack}}} & (1)\end{matrix}$wherein ΔP represents the differential pressure [Pa], μ represents akinetic viscosity coefficient, Q represents the flow rate of the exhaustgas represented in terms of [m³/h], α represents an edge length of thecell, ρ represents a specific gravity of the exhaust gas, V_(trap)represents a filter volume, Ws represents a wall thickness, Kwrepresents a wall gas permeability, K_(soot) represents a gaspermeability of the collected particulate matter layer, W represents thethickness of the collected particulate matter layer, F is a numericalcoefficient (=28.454), L represents an effective filter length, βrepresents the Forchheimer coefficient of the porous wall, ç representsthe inertial loss coefficient of the exhaust gas entering and exitingthe filter.

Next, the mass m_(soot) of the particulate matter collected by the cell21 b is obtained according to

$\begin{matrix}{W = \frac{\alpha - \sqrt{\alpha^{2} - \frac{m_{soot}}{N_{cells} \times L \times \rho_{soot}}}}{2}} & (2)\end{matrix}$wherein m_(soot) represents the mass [g] of the particulate mattercollected, while N_(cells) represents an aperture number of the cell atthe inlet side, and ρ_(soot) represents the density of the collectedparticulate matter.

Thus, a collection amount per unit time, PM [g/h] is obtained bydividing m_(soot) by the time [h] as measured from the previousregeneration of the secondary diesel particulate filter 22A.

Once the mass PM [g/h] of the particulate matter deposited in a unittime is obtained, the concentration of the particulate matter in theexhaust gas, PM_(conc) [g/m³], is obtained by using the flow rate Q2[m³/h] of the exhaust gas passing through the secondary dieselparticulate filter 22A asPM[g/h]=PM _(conc)[g/m³ ]×Q2[m³/h].  (3)

Because the concentration PM_(conc) of the particulate matter in theexhaust gas takes the same value in the secondary exhaust line 21A andalso in the exhaust lien 21, the amount of the particulate matterPM_(enter full filter) [g/h] that has flowed into the diesel particulatefilter 22 is obtained from the mass PM [g/h] of the particulate matterdeposited per unit time, asPM _(enter full filter)[g/h]=PM _(conc)[g/m³ ]×Q1[m³/h]  (4)Further, from this, the amount of the particulate matter deposited inthe filter is obtained by taking into consideration the collectionefficiency of the filter. In the foregoing, Q1 represents the flow rateof the exhaust gas passing through the primary diesel particulate filter(DPF) 22. Q1 may be obtained by actual measurement or estimated from theoperational state of the engine.

FIG. 9 shows the relationship between the differential pressureoccurring across the primary diesel particulate filter (DPF) 22 of theexhaust gas purifying apparatus of the embodiment of FIG. 6 and thedeposition amount of the particulate matter in the primary dieselparticulate filter (DPF) 22, wherein it should be noted that thecontinuous line shows the case in which the deposition amount of theparticulate matter in the main diesel particulate filter 22 is obtainedby using the secondary diesel particulate filter 22A and Equations (1)to (4). On the other hand, the dotted line represents the case in whichthe deposition amount of the particulate matter in the primary dieselparticulate filter (DPF) 22 is obtained directly from the differentialpressure across the primary diesel particulate filter (DPF) 22.

Referring to FIG. 9, it can be seen that there can occur a variation,and hence error, of as much as about ±50% in the differential pressureacross the primary diesel particulate filter (DPF) 22 when compared atthe same deposition amount of the particulate matter.

Contrary to this, it is possible to obtain the amount of deposition ofthe particulate matter collected by the primary diesel particulatefilter (DPF) 22 within the error of about ±10% by obtaining thedifferential pressure ΔP across the secondary diesel particulate matterand by using Equations (1) to (4).

Thus, according to the embodiment of the present invention, it becomespossible to evaluate the deposition amount of the particulate matter inthe primary diesel particulate filter (DPF) 22 in the exhaust gaspurifying apparatus of the embodiment of FIG. 6 precisely by measuringthe differential pressure ΔP formed in the secondary diesel particulatefilter 22A of small volume, and it becomes possible to execute theregeneration of the primary diesel particulate filter (DPF) 22 withoptimum timing by way of carrying out the post injection based on theforegoing result. With this, unnecessary post injection is avoided andthe fuel efficiency of the vehicle is improved.

In the construction of the embodiment of FIG. 6, it is possible to use aknown Vencheri flow meter or hotwire flow meter, wherein the flow meter24 can control the exhaust gas flow rate in the secondary exhaust line21A generally constant within the range of about 50 to about 6000ml/min, for example. With this, one-sided flow of the exhaust gasthrough the secondary exhaust line 21A is avoided, and it becomespossible to obtain the deposition amount of the particulate matter inthe primary diesel particulate filter (DPF) 22 from the depositionamount obtained by using the secondary diesel particulate filter 22A,with further improved precision.

Here, it should be noted that the “differential pressure measuring partmeasuring a differential pressure between an inlet and an outlet of saidsecondary diesel particulate filter” includes not only the differentialpressure gauge that measures the differential pressure between the inletside and the outlet side of the secondary diesel particulate filter 22Abut also the construction that uses a pressure gauge only at the outletside of the diesel particulate filter 22A. With such a construction, thepressure value of the initial state (the state immediately afterregeneration) is memorized and the differential pressure is calculatedby measuring the pressure for the state in which there occurreddeposition of the particulate material in the secondary dieselparticulate filter 22A and by subtracting the pressure value thusobtained from the memorized initial pressure value.

Further, it is also possible to provide a flow meter or a flow velocitymeter at the inlet side and the outlet side or only at the outlet sideof the secondary diesel particulate filter for measuring thedifferential pressure. With such a construction, the differentialpressure is obtained from the reading value of the flow meter, flowvelocity meter, or the like, provided at the inlet side and the outletside of the secondary diesel particulate filter. Alternatively, thedifferential pressure may be obtained from the reading value of the flowmeter or the flow velocity meter at the outlet side of the secondarydiesel particulate filter, by comparing the reading value for theinitial state (the state immediately after regeneration) and the readingvalue for the state where there is caused deposition of the particulatematter in the secondary diesel particulate filter.

The embodiment of the present invention has the feature of obtaining theamount of the particulate matter deposited in the primary dieselparticulate filter (DPF) 22 from the differential pressure obtained forthe secondary diesel particulate filter 22A by using Equation (1), andthus, any instruments including those that are used conventionally formeasuring a differential pressure may be used for measuring thedifferential pressure of the secondary diesel particulate filter.

SECOND EMBODIMENT

FIG. 10 is a flowchart showing the exhaust gas purifying methodaccording to a second embodiment of the present invention that uses theexhaust gas purifying apparatus of the embodiment of FIG. 6.

Referring to FIG. 10, the flow rate in the secondary exhaust line 21A isset to a predetermined value in the range of about 50 to about 6000ml/min in the step 1 by using the flow meter 24, or in some cases byusing the valve 23, and the differential pressure ΔP across thesecondary diesel particulate filter 22A is detected by the differentialpressure gauge 22B. Further, the temperature of the exhaust gas isdetected by using the temperature measuring part T1.

Next, in the step 2, the layer thickness W of the particulate mattercollected by the secondary diesel particulate filter 22A is obtainedfrom the differential pressure ΔP detected in the step 1 according toEquation (1). Here, it should be noted that the temperature T of theexhaust gas may be obtained by using the temperature measuring part T2of the primary diesel particulate filter (DPF) 22 in place of using thetemperature measuring part T1 of the secondary diesel particulate filter22A. Further, the temperature T may be calculated from the temperaturesof the temperature measuring parts T1 and T2 (in the form of averagevalue, maximum value, minimum value, for example). From the viewpoint ofcalculating the amount of the particulate matter more precisely, it ispreferable to use the temperature measuring part T1 of the secondarydiesel particulate filter 22A. For the temperature measuring part, athermocouple may be used, while it is also possible to use anything aslong as it can measure the temperature. While it is preferable tomeasure the temperature of the exhaust gas inside the exhaust pipe, itis also possible to measure the temperature of the filter or the cell.

Further, in the step 2, the mass m_(soot) of the particulate mattercollected by the cell 21 b is obtained from the layer thickness Wdetected in the step 1 by using Equation (2) mentioned previously.

Further, in the step 3, it is judged whether or not the mass m_(soot) ofthe layered particulate matter deposited in the cell 22 b of thesecondary diesel particulate filter 22A has exceeded a predeterminedthreshold Th0, and if the result is NO, the process returns to the step1.

When the mass m_(soot) of the layered particulate matter deposited inthe cell 22 b of the secondary diesel particulate filter 22A hasexceeded the predetermined threshold Th0 in the step 3, the heater 22 his activated in the step 4 and the particulate matter 22 c is removed byburning.

Meanwhile, in the process of FIG. 10, the concentration PM of theparticulate matter in the exhaust gas is obtained in the step 11 fromEquation (3) while using the mass m_(soot) of the collected particulatematter in the cell 22 b obtained in the step 2, and the deposited amountPM_(enter full filter) of the particulate deposited in the principaldiesel particulate filter 22 is obtained from Equation (4) and from thecollection efficiency of the primary diesel particulate filter (DPF) 22.

Thus, in the step 12, it is judged whether or not the deposited amountPM_(enter full filter) of the particulate matter in the primary dieselparticulate filter (DPF) 22 exceeds a predetermined threshold value Th1,and if the result of judgment is NO, the operation returns to the stepS11.

In the event it is judged in the step 12 that the deposited amountPM_(enter full filter) of the particulate matter in the primary dieselparticulate filter (DPF) 22 exceeds the predetermined threshold valueTh1, post injection is executed in the step 13 by controlling an enginecontrol unit (ECU), and the deposited particulate matter in the primarydiesel particulate filter (DPF) 22 is removed by burning. Thereby,regeneration of filter is achieved.

With the process of FIG. 10, it is possible to carry out theregeneration of the secondary diesel particulate filter 22A and theprimary diesel particulate filter (DPF) 22 independently, and thus, itis possible to always maintain the deposited amount of the particulatematter 22 c, or the amount of the soot layer, in the cell 22 b, whichconstitutes the secondary diesel particulate filter 22A, to be a smallvalue of about 0.5 g/l or less. With such a construction, it becomespossible to improve the sensitivity of the particulate matter sensorthat uses the secondary diesel particulate filter 22A.

With the construction of the embodiment of FIG. 6, in which the valve 23is inserted into the secondary exhaust line 21A, there is caused no sucha situation that the exhaust gas flows predominantly through thesecondary diesel particulate filter where regeneration has been madeeven when the regeneration of the secondary diesel particulate filter22A is conducted independently to the primary diesel particulate filter(DPF) 22, and there is caused no error in the evaluation of thedeposited amount of the particulate matter in the primary dieselparticulate filter (DPF) 22.

Thereby, it should be noted that there is no need for the valve 23 tomaintain the exhaust gas flow rate in the secondary exhaust line 21Aexactly at a constant level but it is just sufficient to avoid extremedeviation of the exhaust gas flow to the secondary exhaust line 21A.

Thus, in the second embodiment noted above, the differential pressureΔP, the exhaust gas temperature T and the exhaust gas flow rate Q aremeasured (step 1), the mass of the particulate matter collected by thesecondary diesel particulate filter is obtained by using Equations (1)and (2) from the foregoing result of measurement (step 2), and theamount of the particulate matter collected by the primary dieselparticulate filter is obtained from the amount of the particulate mattercollected in the secondary diesel particulate filter by using Equations(3) and (4) and further using the collection efficiency of the primarydiesel particulate filter (step 11).

In FIG. 10, and also in FIG. 11 to be explained below, the primarydiesel particulate filter (DPF) 22 is designated as DPF while thesecondary diesel particulate filter 22A is designated as sub-DPF.Further, the deposition of diesel particulate matter is designated asDPM depo.

On the other hand, the process of obtaining the amount of theparticulate matter collected in the primary diesel particulate filtermay be modified as shown in FIG. 11.

Thus, in FIG. 11, the process for obtaining the amount of theparticulate matter collected by the primary diesel particulate filter(step 11) is carried out in parallel with the process of obtaining theamount of the particulate matter collected by the secondary dieselparticulate filter (step 2), while using the result of measurementobtained in the step 1.

FIG. 12 shows the overall construction of the diesel engine system thatincorporates therein the exhaust gas purifying apparatus of FIG. 6. InFIG. 12, those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

According to an embodiment of the present invention of FIG. 12, there isprovided an exhaust gas purifying apparatus, comprising: a primarydiesel particulate filter 22 provided in a primary exhaust line 21 of adiesel engine; a secondary exhaust line 21A branched from the primaryexhaust line 21 from a branching point located at an upstream side ofthe primary diesel particulate filter 22; a secondary diesel particulatefilter 22A provided in the secondary exhaust line 21A, the secondarydiesel particulate filter 22A having a soot storage capacity smallerthan the soot storage capacity of the primary diesel particulate filter21; a low pressure part {circle around (1)}, {circle around (2)},{circle around (3)} provided in the secondary exhaust line 21A at adownstream side of the secondary diesel particulate filter 22A, the lowpressure part providing a pressure lower than a pressure of thebranching point; and a differential pressure measuring part 22B (seeFIG. 6) measuring a differential pressure between an inlet and an outletof the secondary diesel particulate filter 22A.

Preferably, the secondary exhaust line 21A has a downstream end part atthe downstream side of the secondary diesel particulate filter 22A suchthat the downstream end part is connected to an air intake part {circlearound (2)} of the diesel engine.

Preferably, the downstream end part is connected to an upstream side{circle around (2)} of an air filter 11AF.

Preferably, the secondary exhaust line 21A has a downstream end part atthe downstream side of the secondary diesel particulate filter 22A suchthat the downstream end part is connected to the primary exhaust line 21at a downstream side {circle around (1)} of the primary dieselparticulate filter 22.

Preferably, the secondary exhaust line 21A has a downstream end part atthe downstream side of the secondary diesel particulate filter 22A suchthat the downstream end part is connected to an exhaust gasrecirculation line {circle around (3)} of the diesel engine 11.

Referring to FIG. 12, the diesel engine 11 includes an air intake systemincluding an air intake line 11in and an exhaust system including aprimary exhaust line 21, wherein an air filter 11AF is provided to theair intake line 11in. Further, there is provided an impeller C of aturbocharger 11T in a part of the air intake line 11in at the downstreamside of the air filter 11AF, wherein the turbocharger 11T is driven bythe exhaust gas and compresses the sucked in air. The air compressed bythe turbocharger 11T is introduced, after being cooled by an air cooler11AC, into the diesel engine 11 with a flow rate controlled by a valve11AV.

The exhaust gas of the diesel engine 11 is emitted to the primaryexhaust line 21 and is emitted, after driving a turbine T of theturbocharger 11T, via an oxide catalyst (DOC) 22Ox and the primarydiesel particulate filter (DPF) 22 of FIG. 6. Thereby, a part of theengine exhaust gas is returned to the air intake line at the upstreamside of the engine from the primary exhaust line 21 by an EGR systemthat includes a valve 21V, for the purpose of NOx reduction.

The air filter 11AF is provided with a sensor S1 for measuring the airtemperature and flow rate in the air intake line 11in and there areprovided sensors S2 and S3 for measuring the sucked air temperature andpressure between the valve 11AV and the engine 11. Further, a sensor S4is provided to the primary exhaust line 21 for measuring the temperatureof the exhaust gas emitted from the oxygen catalyst 22Ox and there isfurther provided a differential pressure sensor (not shown) incorrespondence to the construction of FIG. 6 for measuring thedifferential pressure across the primary diesel particulate filter (DPF)22.

With the construction of FIG. 12, the secondary exhaust line 21Aexplained with reference to FIG. 6 is branched from the primary exhaustline 21 and the secondary particulate filter 22A is provided to thesecondary exhaust line 21A. Thereby, the exhaust outlet of the secondarydiesel particulate filter 22A is connected to any of: {circle around(1)} downstream end of the primary diesel particulate filter (DPF) 22;{circle around (2)} the air intake line 11in at the upstream side of theair filter; {circle around (3)} the part where the pressure is lowerthan the inlet of the secondary diesel particulate filter 21A such as apart of the EGR system, particularly the part between the engine exhaustand the valve 21, and the exhaust gas in the primary exhaust line 21 issucked into the secondary diesel particulate filter 22A. This isequivalent of connection a suction pump to the downstream side of thesecondary diesel particulate filter 22A in the construction of FIG. 6,and it becomes possible to supply the exhaust gas positively to thesecondary diesel particulate filter 22A.

Further, while the explanation heretofore has been made for the case ofusing a honeycomb component of SiC for the primary diesel particulatefilter (DPF) 22 and the secondary diesel particulate filter 22A, theembodiment of the present invention is by no means limited to suchparticular filter components, and it is also possible to use Si—SiC, anitride such as aluminum nitride, silicon nitride, boron nitride,tungsten nitride, or the like, a carbide such as Zirconium carbide,titanium carbide, tantalum carbide, tungsten carbide, or the like, anoxide such as alumina, zirconium oxide, cordierite, mullite, silica,aluminum titanate, or a porous body of metal such as stainless steel.Further, it is possible to use a structural body such as corrugate orelement plate in addition to the honeycomb structure.

The exhaust gas purifying apparatus of the embodiment of the presentinvention has a compact size and is applicable not only to largevehicles such as trucks or industrial machines but also to passengercars.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. An exhaust gas purifying apparatus, comprising: a primary exhaustline exhausting an exhaust gas of a diesel engine; a primary dieselparticulate filter provided in said primary exhaust line of the dieselengine, said primary diesel particulate filter purifying said exhaustgas by capturing a particulate matter in said primary exhaust line; asecondary exhaust line branched from said primary exhaust line from abranching point located at an upstream side of said primary dieselparticulate filter; a secondary diesel particulate filter provided insaid secondary exhaust line, said secondary diesel particulate filterhaving a soot storage capacity smaller than the soot storage capacity ofsaid primary diesel particulate filter; a low pressure part provided insaid secondary exhaust line at a downstream side of said secondarydiesel particulate filter, said low pressure part providing a pressurelower than a pressure of said branching point; and a differentialpressure measuring part measuring a differential pressure between aninlet and an outlet of said secondary diesel particulate filter, whereinsaid exhaust gas purifying apparatus estimates a concentration of saidparticulate matter in said exhaust gas based on said differentialpressure and estimates the amount of particulate matter flowing intosaid primary diesel particulate filter based on said concentration ofsaid particulate matter.
 2. The exhaust gas purifying apparatus asclaimed in claim 1, wherein said secondary exhaust line has a downstreamend part at said downstream side of said secondary diesel particulatefilter such that said downstream end part is connected to an air intakepart of said diesel engine.
 3. The exhaust gas purifying apparatus asclaimed in claim 2, wherein said downstream end part is connected to anupstream side of an air filter.
 4. The exhaust gas purifying apparatusas claimed in claim 1, wherein said secondary exhaust line has adownstream end part at said downstream side of said secondary dieselparticulate filter such that said downstream end part is connected tosaid primary exhaust line at a downstream side of said primary dieselparticulate filter.
 5. The exhaust gas purifying apparatus as claimed inclaim 1, wherein said secondary exhaust line has a downstream end partat said downstream side of said secondary diesel particulate filter suchthat said downstream end part is connected to an exhaust gasrecirculation line of said diesel engine.
 6. The exhaust gas purifyingapparatus as claimed in claim 1, wherein said secondary exhaust linefurther includes a flow meter or equivalent meter.
 7. The exhaust gaspurifying apparatus as claimed in claim 6 further including a holder,and wherein said flow meter or equivalent meter is accommodated in saidholder.
 8. The exhaust gas purifying apparatus as claimed in claim 1,wherein said secondary exhaust line further includes a temperaturemeasuring part.
 9. The exhaust gas purifying apparatus as claimed inclaim 8 further including a holder, and wherein said temperaturemeasuring part is accommodated in said holder.
 10. The exhaust gaspurifying apparatus as claimed in claim 1, wherein said secondary dieselparticulate filter includes a heater.
 11. The exhaust gas purifyingapparatus as claimed in claim 1, wherein said secondary exhaust lineincludes a valve that maintains a flow rate of said exhaust gas flowingtherethrough at a predetermined value.
 12. The exhaust gas purifyingapparatus as claimed in claim 1 further including a holder, and whereinat least one of said differential pressure measuring part, and saidsecondary diesel particulate filter is accommodated in said holder.